Pakistan's detained opposition leader Benazir Bhutto has called for President Pervez Musharraf to step down.

Ms Bhutto made the call after police mounted a massive security operation to prevent a protest march in Lahore, where she is under house arrest.

It was her most direct challenge to Gen Musharraf's rule and was welcomed by key opposition figures - including ex-PM Nawaz Sharif and Imran Khan.

They told the BBC it was meaningless to hold elections under emergency rule.

Mr Sharif said he was looking forward to co-operating with Ms Bhutto - and would back an election boycott if Gen Musharraf refused to lift restrictions and "restore the judiciary".

"We should now be looking towards the greater cause, which is to save the country from a catastrophic situation," he said.

Imran Khan, leader of the Movement for Justice Party, said he and Ms Bhutto had agreed on the telephone that there should be no negotiations with Gen Musharraf, just a call for free and fair elections.

I think it's time for General Musharraf to leave. He is totally out of his depth

"I am glad that all the opposition parties have come round to our point of view, which is that there should be no compromise with a military dictator," he added.

Pakistan's government has refused to bow to pressure to lift emergency rule, which was imposed on 3 November on the grounds of rising militancy and "interference" by the judiciary, but has attracted growing international condemnation.

The US Deputy Secretary of State, John Negroponte, will travel to Pakistan later this week to urge Gen Musharraf to lift the state of emergency and hold free elections, a state department spokesman has said.

Bundled away

Earlier, police in Lahore imposed a seven-day detention order on Ms Bhutto, ahead of a planned "Long March for Democracy" to demand the end of emergency measures.

Hundreds of activists were arrested in and around the eastern city as police strengthened the cordon around Ms Bhutto's home.

Vehicles, barbed wire and metal barriers surrounded the house where Ms Bhutto was staying, and thousands of police were deployed around the city.

If [Bhutto's supporters] try to take the law into their own hands, then we will resort to all means necessary

The former prime minister extended her previous demands for free elections and an end to emergency rule, and for the first time called for Gen Musharraf to quit.

In interviews with the BBC, she said the Pakistani people had lost confidence in Gen Musharraf's ability to steer the country towards democracy.

"It's time for him to leave," she said. "He has lost the confidence of the people of Pakistan... He is unable to give the nation a fair election... He is bent on maintaining and sustaining a dictatorship," she said.

She confirmed there were "no circumstances" in which she would serve as prime minister if Gen Musharraf remained president.

The BBC's Barbara Plett in Lahore says Ms Bhutto is sounding more like the other opposition activists who all along have said that it is Gen Musharraf who is the problem - but people still wonder if she is just toughening her rhetoric to put pressure on him.

In addition to gaining the support of Mr Sharif and Mr Khan, Ms Bhutto contacted Qazi Hussain Ahmad, the leader of Pakistan's main coalition of radical Islamist parties, the United Action Front, according to the AFP news agency.

"I want to build an alliance, a single-point agenda for the restoration of democracy," AFP quoted her as saying.

Ms Bhutto urged supporters to go ahead with the 270km (170-mile) march from Lahore to Islamabad without her, but there is little evidence of anything more than sporadic attempts by small groups, our correspondent says.

Until now, he reports, Washington's preferred option appeared to be an arranged "political marriage" between Ms Bhutto and Gen Musharraf - who President George Bush has identified as a key partner in the war on terror.

But reports from Washington indicate that the Americans too are now losing patience with the general, our correspondent says.